Evaporites, brines and base metals: Fluids, flow and 'the evaporite that was'

• Published in: Australian journal of earth sciences 1997-04, Vol.44 (2), p.149-183
• Main Author: Warren, J. K.
• Format: Article
• Language: English
• Subjects: base metals; brines; evaporite; geochemistry; Marine; meta-evaporite
• ISSN: 0812-0099; 1440-0952
• DOI: 10.1080/08120099708728302

Waters in modern evaporite systems are marine, non-marine, or hybrid but mineralogies in most ancient systems are not so simple that marine and non-marine brines can be easily interpreted from the chemistry of their precipitates. Complications arise related to subsurface brine mixing and back-reactions both at the surface and in the subsurface. The precipitation order of ancient bittern salts from seawater may have been dependent on flux rates of river inflow relative to flux rates through mid-ocean ridges. In ancient continental systems the chemistry of the inflow waters was a fundamental control on the subsequent mineral paragenesis. Our lack of hydrogeochemical understanding of ancient evaporite systems has led to the 'potash problem'. Potash evaporites, traditionally interpreted as marine salts, fall into two categories: (i) potash deposits characterised by MgSO 4 salts, such as polyhalite, kieserite and kainite; and (ii) potash deposits characterised by assemblages containing halite, sylvite and carnallite and entirely free or very poor in the magnesium-sulfate salts. This latter group makes up more than 60% of ancient potash deposits. The former group may well be marine-derived but the latter group must have precipitated from Na-Ca-Mg-K-Cl brines with compositions quite different from that of concentrated modern seawater. After primary precipitation, ongoing pore-water flow propels near-surface and burial diagenesis, both processes that can dissolve and reprecipitate evaporites and drive the chemical evolution of subsurface brines. The hydrologic framework of a large evaporite basin consists of several regimes: (i) the active phreatic-depositional; (ii) the compactional; (iii) the thermobaric; and (iv) the active phreatic-exhumation/uplift. Boundaries are indistinct and transitional. Nonetheless the regimes show hydrological end-members characterised by distinct relative positions, hydrogeochemistries, flow dynamics and evaporite textures. Basin-scale dissolution and remobilisation of evaporite units can occur within all these hydrological settings with the relatively impervious evaporite unit acting both as a focus for fluid flow and as a source of dissolved ions in the subsurface brines. On a geologic time scale both meteoric and brine-reflux driven circulations are rapid and at least partially convective through the subsiding sediment pile. Evaporite units can also act as pressure seals and instigate convectional flow in the compactional and therm